Field
The present invention relates generally to the field of the detection and measurement of radiation and in particular, though not exclusively, to a method and apparatus for the recovery, from a radiation detector, of data affected by pulse pile-up.
Description of the Related Technology
The accurate detection and measurement of radiation is employed in many industries including homeland security, scientific instrumentation, medical imaging and the minerals processing industry. These and other industries use the detection and measurement of radiation for the non-invasive analysis of materials or other specimens. Transmission based imaging, spectroscopic analysis or other modalities can be used to perform such analysis.
Spectroscopy, for example, is commonly used to analyze materials. Knowledge about the material is obtained by analysis of radiation emission from elements within the specimen. This emission of radiation can be stimulated emission due to some form of incident radiation or the result of natural emission from the constituent elements.
Gamma-ray spectroscopy, for example, is a form of spectroscopy in which the emitted electromagnetic radiation is in the form of gamma-rays. In gamma-ray spectroscopy the detection of the resulting radiation is commonly performed with a scintillation crystal (such as thallium-activated sodium iodide, NaI(Tl)), though there are a number of other detector types that can also be used. NaI(Tl) crystals generate ultra-violet photons pursuant to incident gamma-ray radiation. These photons may then be directed to a photomultiplier tube (PMT) which generates a corresponding electrical signal or pulse. As a result, the interaction between the photons and the detector gives rise to pulse-like signals, the shape of which is determined by the incident gamma-ray radiation, the detecting crystal and the PMT. The fundamental form of these pulse-like signals is referred to as the impulse response of the detector.
The output from the photomultiplier is an electrical signal representing the summation of input signals, of determined form, generated in response to discrete gamma rays arriving at the scintillation crystal. By examining the detector output over time, and in particular the amplitude of the component signals, it is possible to deduce information regarding the chemical composition of the material.
Analysis by gamma-ray spectroscopy requires the characterization of the individual signals generated in response to incident gamma-rays. Signal parameters of particular interest include signal amplitude, number and time of occurrence or temporal position (whether measured as time of arrival, time of maximum or otherwise). If the arrival times of two gamma-rays differ by more than the response time of the detector, analysis of the detector output is relatively straightforward. However, in many applications a high flux of gamma-rays cannot be avoided, or may be desirable so that spectroscopic analysis can be performed in a reasonable time period. As the time between the arrivals of gamma-rays decreases, characterization of all resultant signals becomes difficult.
In particular, the analysis is affected by a phenomenon known as pulse pile-up [G. F. Knoll, Radiation Detection and Measurement, 3rd edition, Chapter 17, pp. 632-634, 658 and 659, John Wiley and Sons, New York 2000], whereby multiple gamma-rays arriving more or less simultaneously produce signals which sum together and may be counted as a single signal. The magnitude of this combined signal is greater than the individual components, leading to errors in later analysis.
The energy of an incident gamma-ray may be reflected in the amplitude of the pulse-like signal produced by the detector. The presence of specific gamma-ray energies within the detector signal is indicative of particular elements in the material from which gamma-rays originate. Thus, a failure to differentiate a large amplitude signal caused by a single scintillation event from the superposition of multiple events can have a serious effect on the accuracy of subsequent spectroscopic analysis.
Some existing techniques aim to prevent corruption of the spectroscopic analysis due to pulse pile-up. Certain pulse shaping electronics have been shown to reduce the response time of the detector resulting in a diminished prevalence of pile-up in the final spectrum [A. Pullia, A. Geraci and G. Ripamonti, Quasioptimum γ and X-Ray Spectroscopy Based on Real-Time Digital Techniques, Nucl. Inst. and Meth. A 439 (2000) 378-384]. This technique is limited, however, by detector response time. Another approach is ‘pulse pile-up rejection’ whereby signals suspected to contain pulse pipe-up are discarded. Only signals free from pulse pile-up are used in spectroscopic analysis. However, as the rate of radiation incident on the detector increases, so too does the likelihood that pulse pile-up will occur and the more it is necessary to discard data. Accordingly, existing pulse pile-up rejection is of limited usefulness since a state is quickly reached beyond which a higher incident radiation flux ceases to reduce the time needed for analysis, as an increasing percentage of data must be rejected.
A more sophisticated approach is to make use of prior knowledge about the profile of a single pulse from the detector or to model mathematically the parameters of a signal. It is then possible in principle to distinguish signals or pulses that originate from a single event from those caused by pulse pile-up. In one such method of analysis [R. J. Komar and H.-B. Mak, Digital signal processing for BGO detectors, Nucl. Inst. and Meth. A 336 (1993) 246-252], signals that depart from the simple profile are selected for subsequent analysis. This analysis involves fitting, via an iterative process, two pulses of varying separation and amplitude. Once the fit has been determined, the characteristics of the individual pulses are known from the fitting parameters and hence a pulse arising from two closely occurring signals can be decomposed into the corresponding discrete signals. However, this approach fails to accommodate circumstances where pulse pile-up is caused by the superposition of more than two signals. The iterative optimization is computationally expensive and the time taken to carry out this procedure renders it impractical in most situations.